The present invention relates to a process for producing grinding elements in a casting mold, the process including the use of a cold hardening or setting synthetic resin as the binder in a mixture of abrasive grains and binder to which additives have possibly been added.
Such processes are employed in the production of hard grinding elements, i.e. grinding wheels, ring wheels, cup wheels, honing tools, etc.
In the abrasive producing industry, the view is represented that binders for such hard ("hard" to be understood here in contrast to elastomer) grinding elements which have high abrasive qualities can only be inorganic or duroplastic binders which have a high heat resistance because of the temperatures encountered during grinding, even with cooling.
The known, hard grinding elements are produced with the use of ceramic, phenolic resin, magnesite or epoxy resin binders (the less frequently used binders, such as metal, silicate, polyester and other binders are not considered here).
Vitrified bond grinding elements are used primarily for precision grinding. Due to the required firing for several days at temperatures above 900.degree. C., their manufacture is expensive. Moreover, because of the shrinkage which occurs during firing, these elements must be considerably over-dimensioned, which necessitates corresponding work afterwards.
Phenolic resin bonds are based on the simultaneous use of phenolic resols and phenolic novolaks. Aside from the necessary hygienic measures during manufacture, the hardening process, which takes place at temperatures around 175.degree. C. for a period of up to two days, also constitutes a threat to the environment since, in addition to water, considerable quantities of free phenol, formaldehyde and ammonia are released during this process. During cleaning of the mixers, such substances enter into the waste water together with the solvents, which necessitates complicated cleaning.
Both above-described types of bonds have the characteristic that the structure of the grinding element is porous, and in particular is very porous in the region of the coarse grains and less porous in the region of the fine grains. It is difficult, if not impossible, to set the density of the abrasive grains in such grinding elements over the entire grain size spectrum according to the specifically intended use, and particularly to provide a sufficient chip space volume in the region of the fine and finest grain sizes.
Additionally, phenolic resin bonds have the drawback that they have low resistance to the alkali coolants, so that this type of bond is used primarily only for dry grinding.
Grinding elements in magnesite bonds, in contrast to those in ceramic or phenolic resin binders, are cast and thus dense, i.e. practically free of pores. Nevertheless they offer extremely cool grinding, particularly for hardened steels, and have a high abrasive output. Therefore, such grinding elements are used primarily to sharpen knives, scissors, nippers and other tools, the ends of spiral springs and the like.
The drawbacks of grinding elements in magnesite bonds are many. For example, they can be used only for circumferential operating speeds up to 20 m/sec. Additionally, they change their hardness over time, so that they can be used to optimum efficiency only during a period from one to four months after manufacture. A further significant drawback is the magnesium chloride released during grinding, which leads to extensive corrosion, particularly on the protective hoods of the machines, and constitutes a considerable contamination of the waste waters.
Because of these drawbacks of magnesite bonds, epoxy resin bonds have increasingly been introduced in recent years, particularly in the cutlery industry, where they are employed with lower abrasive outputs and in the fine grain range. One handicap of epoxy resins is their high viscosity. Although basic resins containing a large amount of reactive diluter are available with viscosities of about 1000 mPa.s, they have insufficient heat resistance due to being diluted. Therefore, such grinding elements always constitute a compromise between just sufficient castability, resin proportion (which inevitably lies at 40 weight percent and higher), heat resistance and performance. Due to these necessary compromises with respect to heat resistance, such grinding elements can be used only for wet grinding.
Moreover, there are the hygienic dangers which exist when working with the primarily cold hardening resin systems due to the organic amines, epichlorohydrin remainders and reactive dilutants in the epoxy resin. Finally, the use of raw materials and the costs connected therewith are very high for grinding elements in epoxy resin bonds.